Prionobelum, Verhoeff, 1924

Donworth, Patrick & Wesener, Thomas, 2024, The first record of the giant pill-millipede genus Prionobelum Verhoeff, 1924 from Thailand, with the integrative description of two species (Diplopoda, Sphaerotheriida, Zephroniidae), Zootaxa 5419 (4), pp. 545-562 : 559

Status of Prionobelum and Cryxus

Our barcoding study highlights that the generic status of Prionobelum , especially vis-a-vis the Malaysian genus Tigridosphaera , the widespread genus Sphaeropoeus and the monotypic Cryxus , is still ill-defined. However, we completely lack any molecular data of Sphaeropoeus and are limited to a single fragment of a mitochondrial gene in the other genera, making any phylogenetic conclusions impossible. Morphologically, Prionobelum is well-defined, based on the anterior telopod: in Sphaeropoeus species the process of podomere 2 forms a weak chela with a process on podomere 3, rather than with the apex (see Wesener 2016b), while in Prionobelum the process and the apex (strongly bent backwards and sharp-edged) form a chela. Other character states have not been explored for the majority of Prionobelum species: they seem to lack additional apical spines on the walking legs, as well as the extended femur found in some (but by far not all!) Zephronia and Sphaerobelum species. Prionobelum species seem to vary considerably in their antenna, antennomere 6 being either axe-shaped or (sub)cylindrical. Prionobelum shares with Sphaerobelum the often incompletely separated and sometimes completely fused podomeres 3 and 4 of the anterior telopod. A white membranous spot visible near the tip of the immovable finger of the posterior telopod is present in some species of Prionobelum (e.g. P. hainani (Gressit, 1941) and P. inthanonense n. sp.), as it is in an unusual group of two Sphaerobelum species only tentatively placed in the latter genus ( Rosenmejer et al. 2021; Bhansali & Wesener 2022). The apparently greatly enlarged operculum of the female vulva observable in some Prionobelum species ( Fig. 3C ) seems to resemble such a structure observed in continental Sphaeropoeus Brandt, 1833 species ( Semenyuk et al. 2020).

Figures

FIGURE 3. Prionobelum inthanonense Donworth & Wesener n. sp. holotype ♂ (MHNG 4E-2) and paratype ♀ (MHNG 4E- 2B), drawings, A. Left leg 9, posterior view. B. Right midbody pleurite; C. Second coxa and prefemur with vulva, posterior view. D. Left anterior telopod, anterior view. E. Left anterior telopod, lateral view. F. Left anterior telopod, posterior view. Abbreviations: as = apical spine; cl = claw; ct = crenulated teeth; Cx = coxa; Fe = femur; O = operculum; Po = postfemur; Pre = prefemur; Sp-p = second podomere process; syn = syncoxite = tarsus; Ti = tibia; vs = ventral spines. Scale bars = 1 mm.